4.1 Solid Waste Characterization & RCRA Subtitle D

Municipal Solid Waste Characterization

Understanding the physical and chemical characteristics of Municipal Solid Waste (MSW) is the fundamental first step in solid waste engineering. The composition of MSW heavily influences the design of collection systems, material recovery facilities (MRFs), waste-to-energy (WTE) plants, and landfills.

Physical Composition

The physical composition of MSW is typically broken down by weight or volume into distinct categories. In the United States, the average MSW stream consists primarily of the following components (by weight before recycling):

• Paper and Paperboard: Historically the largest component, often comprising 25-30% of the waste stream. It is highly combustible and recyclable.
• Food Waste: Constituting roughly 15-20%, this fraction is highly putrescible, contributes significantly to landfill gas (LFG) generation, and has high moisture content.
• Plastics: Making up 12-15%, plastics have a high heating value but do not decompose biologically in a landfill.
• Yard Trimmings: Comprising about 10-15%, yard waste is often composted and is highly variable depending on the season and region.
• Metals, Glass, and Wood: These materials make up the remainder of the waste stream. Metals and glass are non-combustible and non-biodegradable but are highly recyclable.

Moisture Content

Moisture content is a critical parameter because it affects the weight of the waste (impacting collection and transportation costs) and the feasibility of thermal treatment (WTE). Moisture content is typically expressed on a wet-weight basis:

$M_w = \left( \frac{W - D}{W} \right) \times 100$

Where:

• $M_w$ = Moisture content (% wet basis)
• $W$ = Initial (wet) mass of the sample
• $D$ = Final (dry) mass of the sample after drying at 105°C

Typical moisture contents vary widely: food waste can be 70-80% water, while paper is typically 5-10% and plastics are less than 2%.

Ultimate Chemical Analysis

For biological and thermal treatment processes, the elemental composition of the waste is required. Ultimate analysis determines the mass percentages of Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S), and ash. This is often represented by a generalized empirical formula for the organic fraction of the waste: $C_a H_b O_c N_d S_e$.

This formula allows engineers to calculate the stoichiometric oxygen required for complete combustion and the theoretical yields of landfill gas (methane and carbon dioxide) during anaerobic decomposition.

Heating Value

The energy content of solid waste is expressed as its heating value, usually in Btu/lb or MJ/kg. The Higher Heating Value (HHV) represents the total energy released during combustion, assuming the water vapor in the exhaust condenses and releases its latent heat. The Lower Heating Value (LHV) assumes the water remains in the vapor phase, which is more representative of actual incinerator operation. The Modified Dulong Formula is often used to estimate HHV from ultimate analysis data:

$\text{HHV (Btu/lb)} = 145.4 \times C + 620.28 \times \left( H - \frac{O}{8} \right) + 40.5 \times S$

Where C, H, O, and S are the mass percentages of carbon, hydrogen, oxygen, and sulfur, respectively.

Worked Example: A dried waste sample has the following ultimate analysis by mass: 45% C, 6% H, 35% O, and 0.5% S. Calculate the HHV.

• $C = 45$, $H = 6$, $O = 35$, $S = 0.5$
• $\text{HHV} = 145.4(45) + 620.28(6 - (35/8)) + 40.5(0.5)$
• $\text{HHV} = 6543 + 620.28(1.625) + 20.25$
• $\text{HHV} = 6543 + 1007.95 + 20.25 = 7571.2 \text{ Btu/lb}$

RCRA Subtitle D Location Restrictions

The Resource Conservation and Recovery Act (RCRA) Subtitle D establishes federal criteria for Municipal Solid Waste Landfills (MSWLFs). These regulations, codified in 40 CFR Part 258, impose stringent restrictions on where landfills can be located to protect human health and the environment.

1. Airports

Landfills attract scavenging birds, which can cause bird strikes with aircraft. MSWLFs located within 10,000 feet of an airport runway used by turbojet aircraft, or within 5,000 feet of a runway used by piston-type aircraft, must demonstrate that the facility is designed and operated so that it does not pose a bird hazard to aircraft.

2. Floodplains

Landfills located in a 100-year floodplain must demonstrate that they will not restrict the flow of the 100-year flood, reduce the temporary water storage capacity of the floodplain, or result in the washout of solid waste.

3. Wetlands

New MSWLFs and lateral expansions are prohibited in wetlands unless the owner or operator can make several demonstrations, including that no practicable alternative is available, construction will not cause violations of state water quality standards, and there will be no net loss of wetlands (via mitigation).

4. Fault Areas

New MSWLFs and lateral expansions cannot be located within 200 feet of a fault that has had displacement in Holocene time (approximately the last 11,000 years), unless an alternative setback distance is proven safe.

5. Seismic Impact Zones

If located in a seismic impact zone (an area with a 10% or greater probability that the maximum expected horizontal acceleration in hard rock will exceed 0.10g in 250 years), containment structures (liners, leachate collection systems, surface water control systems) must be designed to resist the maximum expected horizontal acceleration.

6. Unstable Areas

Owners must demonstrate that engineering measures have been incorporated into the facility's design to ensure the structural integrity of the structural components in unstable areas, taking into account on-site or local soil conditions, geologic features (like karst topography), and human-made features (like underground mines).